In the process of flotation, the recovery of micro-particle molybdenite is difficult, and it is easy to lose in the tailings. In order to analyze the flotability difference of molybdenite with different particle sizes and to reveal the reasons for the flotability of fine-grained molybdenite, using four particle sizes molybdenite respectively -150+74, -74+45, -45+38 and -38 μm as the research object, through molybdenite flotation tests of pure minerals, first by means of X ray diffraction analysis (XRD) and scanning electron microscopy (SEM) analysis, the floatability of particle size molybdenite and its crystalline characteristics, external morphology and variation were systematically studied and compared. The results showed that the floatability of molybdenite decreased with the decrease of particle size, and the flotation effect deteriorated. The floatability of molybdenite was determined by the area ratio of non-polar, low energy, hydrophobic “surface”and lively, hydrophilic “edge”. In the process of transformation from coarse to fine particle size under the action of external force, firstly, it mainly broke along the cleavage plane, and then broke more along the fracture surface, resulting in the decrease of the area ratio of “surface” and “edge”, and the decrease of the floatability. Meanwhile, the crystalline structure of molybdenite changed from 3R type to 2H type. In addition, compared with 3R type coarse particle size molybdenite, fine particle size molybdenite was mainly 2H type and the surface spacing of (002) crystal surface increased gradually. So water molecules was easy to enter the crystal structure and formed hydrated film on the mineral surface, resulting in an increase of the hydrophilicity of molybdenite. The physical properties of molybdenite with different particle sizes changed by different crystal types. The increase of the distance between the surfaces made the number of hydrophilic molybdenum sulfur bonds exposed to the fracture surface of molybdenite increase, which was the essential reason for the enhancement of hydrophilicity and the decrease of floatability of fine grained molybdenite.